Water, Carbon & Climate Change (AQA A Level Geography)

Positive and negative feedback loops

• All of Earth's systems have feedback mechanisms to maintain dynamic equilibrium
• A feedback loop is a type of chain reaction, where one process leads to another process, leading to another process etc.
• These processes are either negative or positive:
  • Negative feedback helps maintain balance in a system e.g. plant net primary productivity increases due to increased uptake of CO2 for photosynthesis
  • Positive feedback reinforces/amplifies change and pushes a system to its tipping point e.g. enhanced greenhouse effect

Water cycle feedback loop

• The water cycle involves the exchange of energy, which leads to temperature changes
  • As water evaporates, energy is taken from its surroundings which cools the environment
  • When it condenses, it releases energy and warms the environment, this exchange of energy influences the local climate
• Changes to temperatures have reduced Arctic ice and the albedo effect
  • Ice melt in the Arctic releases fresh water into the ocean and this changes the thermohaline pattern 
  • This circulation pattern relies on heavy salt water to transport water, carbon and heat around the globe
  • Large deposits of freshwater into the oceans will disturb the ocean conveyor belt 
  • This has the potential of stalling or reversing the ocean circulation pattern
• Dark oceans absorb more heat, which increases evaporation into the atmosphere - a positive feedback loop
  • However, increased evaporation also increases cloud formation, which radiates insolation back out of the atmosphere which creates a cooling effect - negative feedback
  • More water held in the atmosphere, then higher water volume leads to more precipitation
  • This is because the atmosphere can retain more moisture with higher temperatures

Carbon cycle feedback loop

• Warmer atmospheric temperatures:
  • Extends plant-growing season, this increases carbon absorption from the atmosphere and increases carbon sink - negative feedback
  • Plant growth is limited, even with rising CO2, as plants need other elements such as nitrogen etc.; plus rising temperatures reduce the availability of water and plant growth is reduced
  • Warmer temperatures increase permafrost melting, releasing methane/carbon back into the atmosphere and driving enhanced greenhouse effect - positive feedback
  • Warming increases the risk of wildfires in forested areas, which will lower their ability to be carbon sinks

  • Overall, this releases more carbon than forests can absorb into the atmosphere - a positive feedback loop leads to further warming

Water cycle/carbon cycle feedback loop

• Permafrost melt triggers methane release into the atmosphere
• Methane is a more potent greenhouse gas than CO2 and acts as a positive feedback loop
• Currently, there are approximately 5 gigatons of methane in the atmosphere
• It is estimated that the Arctic permafrost holds hundreds of gigatons of methane
• Water has the ability to absorb and transfer carbon and oceans absorb 33% of CO2 emitted to the atmosphere
• The bulk of human-generated CO2 has been absorbed by oceans, although this rate of absorption is slowing down
• As ice sheets melt, carbon storage increases, although this process cannot continue indefinitely, as eventually, the pH levels of the oceans will drop creating higher levels of ocean acidification
• Warmer temperatures increase marine phytoplankton populations to a point 
  • Phytoplankton release dimethylsulphide (DMS) leading to increased cloud formation and cloud cover
  • Insolation is reduced to the oceans, reducing temperatures, reducing phytoplankton activity, which will lessen cloud cover over the oceans
  • With rising ocean temperatures, photosynthesis is reduced, as phytoplankton prefer cooler waters
  • Also, increasing ocean acidity means molluscs and shell-forming marine creatures are finding it difficult to extract bicarbonate ions needed to convert into calcium carbonate, which reduces their ability to be a carbon reservoir